1. Field of the Invention
The invention relates to the field of gas discharge displays and more particularly to an improved method and apparatus for multiplex driving of such displays.
2. Description of the Prior Art
Gas discharge displays generally include one or more character positions defined within a gas filled envelope. Each character position includes at least one anode and one or more segmented character forming cathodes. When a potential difference of sufficient magnitude is established between the anode and one or more of the character segment cathodes the gas therebetween (usually neon or a neon mixture) ionizes to produce a visual display of the energized character segments. A familar type of such display includes a plurality of character positions each having a seven-segment character cathode formed on a common substrate. A seven-segment decoder/driver is used to convert an input signal to be displayed into drive signals for energizing appropriate ones of the character cathode segments. Such displays find wide application due to their inherent advantages of high brightness and good visibility, reliability, and a pleasing orange-red display color.
Several techniques for driving gas displays are known. The simplest technique is termed DC drive in which all character positions are on (lighted) at one time. As a consequence, each character requires its own decoder/driver. Although such an arrangement has the virtue of simplicity, as the number of character positions is increased above about four or five, the costs of additional decoder/drivers and associated circuitry makes DC drive less cost effective than the other major type of drive, multiplex drive.
In multiplexed operation, characters in the display are not on at one time (as in DC drive) but rather are individually switched on in some sequence at a high repetition rate. Two or more character positions thus "time-share" a single cathode driving device.
The most common method of multiplexing is to connect all like cathode segments in parallel to one cathode driver and scan the display anodes in one of two ways: sequential scan, where each anode is successively switched on for a brief period, or interlaced scan in which anodes are scanned in any sequence so long as no two adjacent digits are successively energized.
Advantages of multiplexed operation include reduced circuitry requirements and thus reduced costs for the display. One major disadvantage of multiplexed operation of gas discharge displays is that when sufficient potential difference exists between the anodes of adjacent characters, the anode with the lower potential will acts as a cathode for the pair and spurious ionization may cause a cosmetic defect called a streamer to appear between two character positions. Such a condition can also exist between two cathodes. Streamers can also occur when the anode of one character position acts as the anode for an adjacent character position. This condition occurs when insufficient blanking time (time for de-ionization) is allowed between adjacent character anode scans.
Several techniques are known for preventing streamers. To prevent the formation of streamers during sequential scanning, the removal of turn on voltage from, and the application of turn on voltage to, adjacent character positions is separated in time by electrode (anode or cathode) blanking. Blanking creates a "dead time" between the on times of adjacent character positions so that ionization from a deenergized digit can sufficiently decay before the next character position is energized. However, inter-character blanking has the disadvantage of requiring special circuitry for controlling character "on" and character blanking time. Further, the upper frequency of operation is somewhat limited since the scan rate is a function of both character "on" and blanking times.
An alternative technique is interlaced scanning in which character positions are scanned such that no two adjacent character positions are successively scanned. For example, in a five character display, the anodes associated with character positions 1, 3, 5 would be scanned followed by a scan of positions 2 and 4. Interlaced scanning thus increases the distance between successively energized character positions and eliminates the need for blanking, but at the expense of requiring more complex scanning circuitry than is needed for sequential scanning.
A third technique for preventing streamers is known as split-cathode multiplexing. In split-cathode multiplexing character positions are paired and physically isolated (e.g. in separate display packages) from adjacent character pairs. Each pair of character positions shares an anode driver and all anode drivers are addressed simultaneously. The odd and even character positions of each pair are alternately driven by first and second cathode drivers. Since successively energized character pairs are separated by the display envelopes, the need for blanking is eliminated as streamers are a physical impossibility. However, split-cathode multiplexing requires somewhat complex addressing circuitry to simultaneously generate the anode drive signals and alternately actuate the odd and even cathode drive signals. Further, such a scheme is useful only in displays where character pairs can be physically isolated from their neighbors.